build facebook signed request in hmac-sha256 method

RequestMaker.java

//http://en.wikipedia.org/wiki/Base64#URL_applications
//http://stackoverflow.com/questions/11106393/url-safe-base64-in-objective-c
//http://stackoverflow.com/questions/10812140/facebook-signed-request-for-ios-hmac-sha256
public class RequestMaker {
  private static final String KEY = "testkey";
	private static final int BASE64_FLAG = Base64.URL_SAFE | Base64.NO_WRAP | Base64.NO_PADDING | Base64.NO_CLOSE;
	public RequestMaker() {
	}
	
	public String getRequestPayload(String token, String user_id, String expires) {
		return new String(Base64.encode(jsonBuilder(token, user_id, expires).getBytes(), BASE64_FLAG));
	}
	
	public String getRequestHash(String token, String user_id, String expires) {
		return getHmacSHA256(getRequestPayload(token, user_id, expires));
	}
	
	public String getRequest(String token, String user_id, String expires) {
		return getRequestHash(token, user_id, expires) + "." + getRequestPayload(token, user_id, expires);
	}
	
	private String getHmacSHA256(String str, String key) {
		String retVal = null;
		try {
			Mac mac = Mac.getInstance("HmacSHA256");
			SecretKeySpec secret = new SecretKeySpec(key.getBytes(), mac.getAlgorithm());
			mac.init(secret);
			byte[] digest = mac.doFinal(str.getBytes());
			retVal = Base64.encodeToString(digest, BASE64_FLAG);
		} catch (Exception e) {
			e.printStackTrace();
		}
		return retVal;
	}
	
	private String getHmacSHA256(String str) {
		return getHmacSHA256(str, KEY);
	}
	
	private String jsonBuilder(String token, String user_id, String expires) {
		JSONObject json = new JSONObject();
		try {
			json.put("expires", expires);
			json.put("user_id", user_id);
			json.put("algorithm", "HMAC-SHA256");
			json.put("oauth_token", token);
		} catch (JSONException jsone) {
			jsone.printStackTrace();
		}
		return json.toString();
	}
}